This invention relates to electronic cameras and more particularly to the control of electronic cameras.
Electronic cameras are commercially available and in widespread use. The basic technology used in electronic cameras is well developed. Electronic cameras generally include CCD or CMOS light sensing elements. The exposure time of the light sensing elements and the gain of the readout circuitry can be set to control the quality of the images which are acquired by the camera.
Some electronic cameras have a built in exposure meter that detects the amount of light reaching the camera. The appropriate exposure time and gain of the sensing elements is then automatically set at values determined by the signal generated by the exposure meter.
Some electronic video cameras which are designed for integration into a personal computers provide the user with a menu such as that shown in FIG. 1 which can be used to set various parameters to control the operation of the camera. A user typically sets the various parameters so that the camera will produce an image that is visually appealing to a human observer. A program in the computer adjusts the actual settings in the camera in a direction which attempts to have the images meet the specifications set by the user. This type of adjustment may take place over several sequential frames. Furthermore as ambient conditions change, the actual exposure time and gain of the sensing elements is varied between frames in an effort to meet the specifications set by the user.
Electronic cameras can be used as part of systems such as that shown in copending applications Ser. No. 09/314,648, 09/343,104, 60/158,015, filed Nov. 3, 1999. The systems shown in the referenced copending applications are designed to detect a digital watermark in an image acquired by a video camera. In such systems after a video camera is used to acquire an image, the acquired image is digitally processed to detect a digital watermark that is contained in the image. It has been found that the optimal settings for a camera used in such systems is not necessarily the optimal settings that are established to provide a pleasing image to a human viewer.
The type of parameters that can be established using a menu such as that shown in FIG. 1 do not always produce optimal conditions for detecting a digital watermark in an acquired image.
The present invention provides a mechanism for automatically setting the exposure time and gain of an electronic camera so that the acquired image has improved characteristics for the detection of a digital watermark. With the present invention the exposure time and gain of an electronic camera are set by first directing the camera at an image with the camera""s controls set to a set of default or initial values. The system then calculates and stores data which represents a histogram of the luminance values of the pixels in the acquired image. Next characteristic values from the histogram data such as the peak value, the centroid value, the RMS value, the median value, etc., are calculated. One or more of the characteristic values of the histogram data are then applied as an index or pointer to values of pre-established settings that are stored in a table. Values from the table are used to set the exposure time and/or gain of the camera. In some embodiments, the variance of the luminance histogram data is also measured and used to index data from the table. In some embodiments, a test target is used to acquire an initial image. In other embodiments, the initial image is made by pointing the camera at the image which contains the watermark data.